Implantable neuromodulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., Arrhythmias). Spinal Cord Modulation (SCM) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as Angina Pectoralis and Incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and Epilepsy. Further, in recent investigations Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
Each of these implantable neuromodulation systems typically includes at least one neuromodulation lead implanted at the desired stimulation site and an Implantable Pulse Generator (IPG) implanted remotely from the stimulation site, but coupled either directly to the neuromodulation lead(s), or indirectly to the neuromodulation lead(s) via one or more lead extensions. Thus, electrical pulses can be delivered from the neurostimulator to the electrodes carried by the neuromodulation lead(s) to stimulate or activate a volume of tissue in accordance with a set of stimulation parameters and provide the desired efficacious therapy to the patient. The neuromodulation system may further comprise a handheld remote control (RC) to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. The RC may, itself, be programmed by a technician attending the patient, for example, by using a Clinician's Programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon.
IPGs are routinely implanted in patients who are in need of Magnetic Resonance Imaging (MRI). Thus, when designing implantable neuromodulation systems, consideration must be given to the possibility that the patient in which neurostimulator is implanted may be subjected to electro-magnetic fields generated by MRI scanners, which may potentially cause damage to patient tissue, malfunction or damage or the neurostimulator, and/or discomfort to the patient.
In MRI, spatial encoding relies on successively applying magnetic field gradients. The magnetic field strength is a function of position and time with the application of gradient fields throughout the imaging process. Gradient fields typically switch gradient coils (or magnets) ON and OFF thousands of times in the acquisition of a single image in the presence of a large static magnetic field. Present-day MRI scanners can have maximum gradient strengths of 100 mT/m, and rapid switching times that yield slew rates at or exceeding 200 mT/m/ms, which is capable of generating unintended peripheral nerve stimulation in patients even without the presence of an implantable device. Typical MRI scanners create gradient fields in the range of 1 Hz to 10 KHz, and radio frequency (RF) fields of 64 MHz for a 1.5 Tesla scanner and 128 MHz for a 3 Tesla scanner. Both of these types of applied fields are activated in bursts, which have comparable frequencies to stimulation therapy frequencies.
While conventional IPGs implanted within a patient undergoing an MRI may be reprogrammed or deactivated (e.g., using a clinician programmer) to temporarily shut down for the duration of the MRI, newer versions of IPGs may be switched to an MRI-specific mode that enables a limited functioning of the implanted system during the MRI. In one technique, the stimulation circuitry of the IPG is deactivated, while allowing the IPG to communicate with the RC. In one novel technique described in U.S. Provisional Patent Application Ser. No. 61/612,241, entitled “Neuromodulation System for Preventing Magnetically Induced Currents in Electronic Circuitry,” which is expressly incorporated herein by reference, voltage supply rails of the IPG electronics are increased to prevent electrical energy induced on the stimulation leads by the MRI fields from circulating through the IPG that may otherwise cause damage to the IPG electronics or painful or unintended stimulation to the patient. To make use of this option, the IPG may be manually placed into the MRI-specific mode before undergoing the MRI procedure.
However, the patient or medical personnel may forget to place the IPG in the MRI-specific mode or otherwise deactivate the IPG before the MRI procedure. The patient may not be mentally conscious, in some cases, and therefore may be unable to inform the medical personnel to manually place the IPG in the MRI-specific mode or otherwise deactivate the IPG. By failing to place the IPG in the MRI-specific mode or otherwise deactivate the IPG, the patient may be put at risk of being exposed to unwanted electrical stimulation and/or discomfort or the IPG may be put at risk of damage during the MRI procedure.
There, thus, remains a need to automatically deactivate or place the IPG in the MRI-specific mode without requiring user intervention. Additionally, since MRI-specific modes are, in general, not identical to modes that deliver optimal therapy, it is also valuable that the method of automatically entering an MRI-specific mode have high sensitivity and specificity for MRI.